Menendez, the ranking member on the Senate Foreign Relations Committee, is seeking a third term in the Senate after federal prosecutors dropped charges of corruption against him earlier this year.

While he remains a big favorite in the New Jersey race, the legal case has dented Menendez's approval ratings, and a recent poll showed a very tight race against his Republican opponent Bob Hugin, a former CEO of pharmaceutical company Celgene Corp.

But Hugin’s willingness to heavily fund his own campaign, including through advertisements attacking Menendez, coupled with millions in outside money pouring in, could give the New Jersey senator his toughest fight to date in a state Democrats had been counting on for an easy win.

Senate Democrats are defending 25 seats in the Senate this campaign cycle, with 10 of those in states where Trump won in 2016, compared to Republicans who only need to protect eight seats. They will likely need to hold on to all those seats and pick up two more to retake the Senate.

“The advertising that Bob Hugin has put into the race has been effective in terms of swaying public opinion,” said Brigid Callahan Harrison, a longtime political observer in New Jersey and political science professor at Montclair State University.

But Harrison said the impact would be “temporary,” believing Menendez’s support would likely “bounce back” as he spends more on advertising.

Hugin has been highlighting Menendez’s corruption charges in the campaign. In 2015, the longtime staple of New Jersey and Washington politics was indicted on bribery and fraud charges over allegations he traded influence in exchange for gifts from a longtime friend who is a Florida ophthalmologist and political donor.

But his trial ended in a hung jury and federal prosecutors dropped the charges, although the Senate Ethics Committee “severely” admonished Menendez in April for his relationship with the donor and friend.

“Senate Ethics Committee Democrats and Republicans, [said] Sen. Menendez violated the law, abused the power of his office, disgraced the Senate,” Hugin said in an interview with Fox News on Wednesday.

“It’s time for change. We’re going to put New Jersey first.”

Menendez has repeatedly denied any wrongdoing in the case, declaring that he’s been vindicated with the dropped charges.

Still, the legal case has dogged Menendez. A Gravis Marketing poll released Monday found Menendez leading Hugin by just 2 points, 43 percent to 41 percent, with 16 percent of those polled still undecided.

That shows an even tighter margin than a May poll from Fairleigh Dickinson, which had Menendez up by 4 points — and it contrasts sharply with surveys from earlier this year that had the Democratic senator leading by double digits.

The Gravis poll also found that 50 percent disapprove of Menendez and only 40 percent approve. A little over half of voters polled said that Menendez’s trial will either somewhat or heavily impact their vote, while 35 percent said it wouldn’t.

Menendez, who previously served in the House, saw an early sign of voter disapproval after winning the June primary by a lower-than-expected 62 percent of the vote, while his little-known Democratic challenger, who reported no fundraising numbers, garnered 38 percent.

And in Hugin, Menendez faces a challenger with deep pockets. The former Celgene CEO has given $15 million of his own money to his campaign. And he will have more reinforcements from Integrity NJ, a super PAC with ties to former New Jersey Gov. Chris Christie (R) that has raised over $2 million since February, according to filings with the Federal Election Commission (FEC).

Menendez, a former mayor of New Jersey’s Union City, is still favored in the race. New Jersey’s blue hue has gotten even deeper since 2016, with Democratic Governor Phil Murphy easily winning the governor’s mansion in 2017.

Plus, it’s not uncommon for New Jersey Democrats to find themselves in tight races with their Republican candidates close to Election Day.

The senator had a slower start to fundraising late last year, but his fundraising has significantly picked up. He raised about $6.1 million since the beginning of 2017 and has $6.4 million in the bank.

Menendez will also have financial help from outside groups, like the super PAC Patients for Affordable Drug Action, which has raised $3 million.

Menendez and his allies have seized on Hugin’s time as head of Celgene, at a time when high drug prices are becoming a campaign issue for Democrats in the fall.

Patients for Affordable Drug Action has spent $1.5 million in ads attacking Hugin over his ties to Celgene, which paid a $280 million settlement last summer over allegations that it branded cancer drugs for unapproved uses. The company has denied any wrongdoing.

“Bob Hugin made his fortune at Celgene on the backs of cancer patients, repeatedly jacking up drug prices, blocking lower-cost generics, and gaming the system to get rich,” said Menendez campaign spokesman Steven Sandberg.

Hugin has pushed back on this characterization, with a recent ad that features the father of a cancer patient, who highlights how Hugin and Celgene "stepped in" after the insurance company denied coverage of the medication his son needed.

Hugin is also vulnerable because Trump is very unpopular in the state. Hugin initially supported the 2016 presidential bid of Christie, a Trump ally who left office in January as one of the most unpopular governors in the country. Hugin was then Trump’s finance chairman for his New Jersey campaign.

“All anyone needs to know about Bob Hugin is that he is a Trump Republican, who not just gave millions to elect Trump and other right-wing conservatives and causes, but was also Donald Trump’s 2016 N.J. finance chair and delegate to the RNC [Republican National Committee],” said Sandberg.

When pressed during his Fox News interview about whether he wants Trump or Christie to campaign for him, Hugin didn’t name them specifically, but said he’d welcome help from “anybody who believes in the values that I have from New Jersey.”

But Hugin stressed that the race is about himself and Menendez.

“This race is Bob Menendez vs. Bob Hugin,” he said in the interview. “I want everybody to come out and support me as long as it’s about New Jersey issues.”